KT120 SE Tube Amplifier: Final thoughts for now.
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- Опубліковано 21 вер 2023
- Well after some introspection and a few hours of listening, these are still worth pursuing. They sound SOOOO good, so I'm going to go in search of the holy grail and find some "bogey" tubes. For now I have a pair biased up decently and these monoblocks are next level good. Part of the magic are these output transformers, they are a must have if you want to duplicate this project.
This series is a DIY project building a pair of single ended, ultralinear KT120 mono block style vacuum tube audio amplifiers. The goal is a pair of high power single ended amps, using only high quality parts, with as few things in the signal path as possible.
This project is using output transformers from Thermionic Labs, custom made to use with KT120 output tubes, which you can buy here:
www.thermioniclab.com/
They also should have power transformers and chokes for this project available.
You can send me links and messaged at:
www.skunkiedesigns.com/contact
Here is my Patreon, as well as the donation page at my site, if you feel so inclined to help fund some of the future projects for the channel!
www.patreon.com/user?u=87080658
www.skunkiedesigns.com/donate
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Nice to hear how good your amp sounds. :)
Thanks so much for such a comprehensive explanation of the work you have done with these amps. That's why I love you and your channel. Thanks again.
From my tech: "I have used kt120’s & 150’s since they came into being. These are tetrodes and many circuit precautions must be built in to make them operate properly. They must have the correct driver that can deliver sufficient current to keep them stable. My design I have developed a method using a constant current source to keep them stable. Also must have regulated screen grid supply. Have had very minor problems with reliability issues using the 120’s. The 150’s because of poor QC is kind of a Turkey shoot. About 20% returns after a few hours run time."
Good to see some silver lining Stephe. Boy, you have worked so hard on those 120's. Well done, to have reached such a point
Great video again Thank you for sharing your valuable knowledge very inspiring.
You say that in high-end audio people do crazier things than this. But I think you have now really gotten the bug, and this tube rolling journey is just the beginning.
I had my own first redplating experience recently. Two times, same unit (Sansui 1000), different reasons: bad (old) coupling caps, then bad tube. It's quite alarming! Good luck with your search!
Fantastic ❤❤❤❤❤❤
I am excited to see how the Upscale Audio tubes work.
Oh you and me both!
Even with ultralinear, on EL34 or EL84 amps, I use a 1K resistor on the screen. Specially on modern cheap EL84 not build for more than 300V on plate and 250V on screen. Tubes last longer. You are right, that could be the solution to your problem. You could also lower the plate voltage (450 - 475V) (+ resistor on screen if needed) then find the right bias. Thanks for this following video.
I always use 1.2K on the screen in UL, including on this amp.
Yeah, hope we'll be getting over the apocalypse... RealSoon.
Looking forward to seeing these WE 300B, a stuff of legends :).
Morning Steph. The drum you kick with your foot, is a kick drum, I think. It is the big bass drum that uses the foot pedal device to smack the drumhead. Great news on finding working tubes and getting the amazing sound you were shooting for.
Yeh, "kick drum" and or "bass drum"
Now i need to go listen to "when the levee breaks" on my chinese amp and klipsch speakers.... and so does my neighborhood....
Thanks. Very useful observations about biasing and screen voltage in relation to sound....I've got some original GEC KT88s and want to give this design a try with them. Great you've got manufactures upping their game....maybe there's a consultancy job in there somewhere?
I just ordered the OPTs from Thermionic!
I saw they have a whole kit assembled now for this specific build!
www.thermioniclab.com/en/amplifiers-kit/
@@SkunkieDesignsElectronics I would do that but I can't afford that much at one time. They now sell on eBay but pricing is a little different. eBay = higher item cost and less shipping / direct = lower item cost and higher shipping. I may end up buying everything from them but I had to have the OPTs since it can effect the sound so much.
Try some GK 71 directly heated pentodes. They can handle 125w SE run 20v heaters and are quite inexpensive. I built a dual mono amp using Switch mode power supplies with 800v on the plate for 50w output with 3% THD into some 100w guitar amp OPT's. Thoriated Carbon plates that can cop a real hiding and last the distance. I like your analysis it's good to see the passion you have for this craft.
Those look fun :)
You got lemons(tubes), make lemonade. I would sure like to hear those on my Lascala's
good
Great to see you more cheerful Steph and that also gives confidence in the build , i have built mine albeit stopped at the resistor fitting, would you say im ok to go ahead following your schematic shown values and then like you play with biasing, I'd be quite happy with a safety margin around 75ma giving reduced output , my room is not huge and running the chorus 2 speakers is more than ample , as i said yesterday the 120 tubes do bring more to the table even in the blueglow amp , no extra power but bass and clarity is a massive improvement over the kt88 , look forward to your preamplifier now , well done you 100%
try for 80ma and I will try to do the bom soon.
@@SkunkieDesignsElectronics great thank you so much
forgot to say earlier Steph - if you still have your blueglow amp give the 2 poor 120's a go in it and see what you think - mine differs from yours in that i stuck to using the single ecc85 ( albeit i found an amazing ecc865 ) - cant thank you enough for all you do for us guys
@christopherredfern, it looks like you will complete your KT-120s before I do. I also have Chorus II speakers I've been carting across the Country multiple times over the last almost 3 decades. I never had an amp that was worth a crap. That is why I'm determined to build these KT-120s. PLEASE let us know how they sound when you're done.
Thanks for diving so much deeper to figure out a solution to the wacky tubes. I am now thinking of building these amplifier again. So what do you think of trying out the Shuguang Treasure KT-120? I realize this company still hasn't gone back into production but there are some available & not that expensive. I read a couple of articles that said they are better then the Tung Sol.
Where is a good place to get some to try?
So maybe there light in the end of the tunnel here! 😊 Since my speakers are 89db I need some power. I might biamp them though and go class D on the bass driver. That might help. 😊 One other concern is that tubes need to be change from time to time which mean that you need to go through this searching process every time if manufacturing quality does not change.
Yup, all concerns. These should have enough power for 89db speakers but I don't know what the environment is as that also matters.
I'm glad you aren't giving up on these monos! Are you going to release a video of your audio analyser suite testing?
I did an earlier video on the first completed monoblock. I plan to do a floowup but am putting these to the side for now while I search for a good pair of tubes.
@@SkunkieDesignsElectronics Sorry, Stephe - I watch your content within hours of it coming out and this one was so long ago (ha ha) that I'd forgotten I'd seen it (!) so I watched it again. Thanks, and keep 'em coming!
As I mentioned in my previous comment in your other video, Western Electric has said on their website that they are working on some other tubes besides the 300B's available now, but they didn't disclose on their website which ones they are working on.
Interestingly, WE is located in GA, just outside of Chattanooga across the TN state line. Besides making 300B tubes, they have a 300B integrated amp ($15K) with a single 300B per channel with built-in phono pre-amp for both MC and MM (and some solid state circuits to do auto-biasing, LCD display with electronic VU meters, remote control, etc). They also have a monobloc 300B amp ($125K per pair) that produces 80 watts per channel RMS using eight WE 300B's per side, as well as three 6SN7GTBs and a single 6SL7.
I'd love it they also made some sweet sounding 6SN7 and 6SL7 tubes.
One of their buildings literally has the Chattanooga, TN / Rossville, GA, City / State line going through the building. Must need an extra bean counter on staff just for property taxes...
@@adamphillips1263 Each county would send separate property tax bills for the property in their county (property taxes are assessed by counties). The company does not have to calculate the property taxes, the county does that and sends a bill. Same would apply if a building was in 2 counties in the same state.
For all other taxes (like corporate income taxes or remittance of sales taxes), it would probably be the address that is the official place of business, which be GA for WE.
How much input swing in Vpp do you feed the KT120 with ? - In my test setup i had upto 90-95 Vpp for 20W out 5% dist
And how much on the input jacks ?
2X the bias voltage.
Stephe, the SE output transfo from Thermionic is $400. Edcor's 3.75k 25w SE output transfo is $140. Do you think the Thermionic sound that much better than the Edcors?
The prices + shipping i.e. price to your door also varies depending on where you live. What makes sense for someone in Europe might not for the US. Also the Thermionic price I believe is for a pair, Edcor is for a single transformer.
But no, I haven't compared the two back to back, but I do know these Thermionic units sound great! I also am not sure what ma Edcor gaps their's at. They need to be gapped high enough to use in this build. The ma rating of the winding the ma they are gapped to use. If I was gonna use the Edcor, I also would go with the 5K version. Hope this helps!
We are living in a world gone mad.
Hi there. This build series has captured my attention because of both the SE output stage and the use of the KT120 tube type. It appears that this design is particularly demanding of the screen grid (G2) and as a result that electrode is not very happy near your ideal bias point. The datasheet recommends only 225VDC on G2 with 400VDC on the plate in its suggested Class-A1 pentode audio service scenario. At that operating point, G2 flows about 14ma current and 3.15watts of heat are produced (Max G2 Pd = 8 watts). It might be worth considering ditching the UL tap altogether and going with a regulated G2 instead. A pair of 0B2 gas regulators in series, a series resistor, and a tiny cap across the tube (
And going to a pure pentode creates a huge increase in distortion, which then requires a lot of global negative feedback, which then takes up a huge amount of the gain from the driver tube, which requires a total redesign of the amplifier. It would also totally change how the amplifier sounds. That doesn't sound like it will "positively impact the design of the circuit" to me.
The problem I am seeing to a QC control issue given only one side of the plate is trying to red plate, various tubes red plate are different levels and I can't turn the bias up high enough to make both side of the tube red plate. Other folks have told me they can bias their KT120 at 90%+ in UL mode and see no red plating. Also reducing the G2 voltage 100V didn't have any impact whatsoever. Clearly you are free to design and publish your own KT120 SE amp design.
@@SkunkieDesignsElectronics Thanks for the response! You are mostly right. The output stage will be operating as a pentode, however, the local feedback loop (resistor from output plate to input tube plate/output tube grid) completely changes the game. With the inclusion of local feedback, the same type of feedback that the the UL transformer tap delivers to the screen, the output tube will actually move towards triode characteristics. Triode linearity can be achieved without a drop of global feedback! Zener diodes could be used to set the screen voltage, by the way. It might be worth trying out at some point if you are curious. These are just some ideas I had kicking around to help keep these KT120/150 tubes from going nuclear. Do with these thoughts what you will, no worries! I wish you luck finding the "bogey" tubes. Have a nice day!
The thing is, they aren't going nuclear like a "normal" red plating tube would do. They will sit there for an hour with one side of the tube glowing lightly and the bias never moves, nor do they sound bad or distorted, crackling or the other stuff an output tube going nuclear does. Honestly they sound fantastic biased there at 100ma. I just don't believe changing from UL with a 10K resistor (which lowered the screen 100V and changed nothing) to lowering the screen another hundred volts is going to allow these tubes to then be biased 30% more.
@@SkunkieDesignsElectronics I'm wondering - if the bias never moves and they don't sound bad, does it actually hurt anything for the tube to glow? Will tube life be cut short? I have no real-life experience with tubes (yet) but wouldn't it be wonderful if we could still bias these tubes at 100ma and the glowing not be a problem? I'm too ignorant to know if this is a stupid question.
I bet the quality of the output has more to do with the output transformers than the tubes in this case.
Likely so. I will say the KT120 tubes sound much better than the KT88's I have tried.
@@SkunkieDesignsElectronics I used to play the “let’s upgrade the OPTs game”. It makes a night and day difference in so many cases, it’s amazing. You can get that very liquid sound and FR improvements that cheaper OPTs cannot deliver.
Not for sale! I'll build my own when I have the money.
I have also made a similar power amplifier, ecc88 driver, KT120 UL working state, 110 mA current, and there is no red screen problem